Sequential mesa avalanche photodiode capable of realizing high sensitization and method of manufacturing the same

ABSTRACT

A sequential mesa type avalanche photodiode (APD) comprises a semiconductor substrate and a sequential mesa portion formed on the substrate. In the sequential mesa portion, a plurality of semiconductor layers, including a light absorbing layer and a multiplying layer, are laminated by epitaxial growth. In the plurality of semiconductor layers, a pair of semiconductor layers forming a pn junction is included. The carrier density of a semiconductor layer which is near to the substrate among the pair of semiconductor layers is larger than the carrier density of a semiconductor layer which is far from the substrate among the pair of semiconductor layers. In the APD, light-receiving current based on movement of electrons and positive holes generated in the sequential mesa portion when light is incident from the substrate toward the light absorbing layer is larger at a central portion than at a peripheral portion of the sequential mesa portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based upon and claims the benefit of priorityfrom the prior Japanese Patent Applications No. 2001-284039, filed Sep.18, 2001; and No. 2002-218311, filed Jul. 26, 2002, the entire contentsof both of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a sequential mesa avalanchephotodiode and a method of manufacturing the same, and in particular, toa sequential mesa avalanche photodiode having a sequential mesastructure in which, in an avalanche photodiode to be used as a lightreceiving element for converting a light signal to an electric signal inan optical communication network or the like, high sensitization can berealized and the fabrication costs of modularization can be greatlydecreased, and to a method of manufacturing the same.

[0004] 2. Description of the Related Art

[0005] As is well-known, recently, the signal speed of light signalsused in optical communication networks has been made much morehigh-speed.

[0006] In accordance therewith, making the speed more high-speed hasbeen required of light receiving elements built in optical communicationequipment transmitting and receiving such light signals.

[0007] Further, in such light receiving elements, it is required thateven low level light signals can be precisely received.

[0008] As such a light receiving element receiving high-speed and weaklight signals, generally, an avalanche photodiode (hereinafter,abbreviated APD) has been put into practice.

[0009] In such an APD, in a state in which a depletion region is formedby applying reverse-bias voltage to a pn junction formed by a pair ofsemiconductor layers whose conductive types are different from oneanother, when an electromagnetic wave of a light signal or the like isincident from the exterior, a pair of an electron and a positive hole isgenerated.

[0010] Further, this pair of the electron and the positive hole ismultiplied by the avalanche phenomenon in the APD, and taken out asvoltage or electric current to the exterior.

[0011] There are various ways of classifying APDs. When classifyingstructurally, there are a planar type and a mesa type, and whenclassifying by main carrier, there are a positive hole type and anelectron type.

[0012] Here, a sequential mesa structure used regardless of the type ofthe main carrier will be described.

[0013] Generally, in order to aim for making the APD high-speed, themesa type, not the planar type, is generally used as the shape of theAPD.

[0014] This is for decreasing the electric capacity of the APD elementitself in order to make the APD high-speed.

[0015] In order to increase the permissible light-receiving current asan APD element, there is the need to remove the bias of thelight-receiving current density flowing through the interior of the mesaportion.

[0016] Therefore, in a mesa type APD element, the shape of the mesa mustbe made to be isotropic, namely, as shown in FIG. 9B, formed conicallyas viewed from the top surface of a substrate.

[0017] Moreover, in a mesa type APD element, when the shape of the mesais formed to be conical, attention must be paid such that thecrystallinity of the cross-section of the mesa is not damaged.

[0018] Therefore, in a mesa type APD element, when the shape of the mesais fabricated, diffusive wet-etching by an etchant which is notanisotropic is necessary.

[0019] By applying this diffusive wet-etching, the sequential mesashape, which is a shape (generally, conical) in which the mesa diameter(cross-sectional area) widens as it approaches the substrate, can beobtained.

[0020] Accordingly, the sequential mesa type APD is generally used formaking the APD high-speed.

[0021] Further, as APDs using positive holes as the main carrier, thereare an APD in which the above-described pn junction is formed byepitaxial growth, and an APD in which the pn junction is formed by Zndiffusion.

[0022]FIGS. 9A and 9B respectively show a cross-sectional view and anexternal perspective view of a sequential mesa type APD, in accordancewith a prior art, which has a sequential mesa structure and in whichpositive holes are used as the main carrier and the pn junction isformed by epitaxial growth.

[0023] Hereinafter, on the basis of FIGS. 9A and 9B, the structure ofthe sequential mesa type APD according to the prior art will bedescribed.

[0024] Namely, in the sequential mesa type APD according to the priorart, as shown in FIGS. 9A and 9B, an n-type buffer layer 2 a, an n-typelight absorbing layer 3 a, an n-type electric field relaxation layer 4a, an n-type multiplying layer 5 a, and a p-type contact layer 6 b aresuccessively formed by epitaxial growth by using an MOVPE(organometallic vapor phase epitaxial growth) method on an n-typesemiconductor substrate 1 a. Therefore, a conical sequential mesaportion 10 is formed by wet-etching from above.

[0025] Next, after a protective layer 7 is coated on the sequential mesaportion 10, a p electrode 8 contacting the p-type contact layer 6 b isformed.

[0026] Further, at the both sides of the sequential mesa portion 10, ann electrode 9 is attached, via a protective layer 11, to another mesaportion formed for attaching electrodes.

[0027] As shown by the arrow in FIG. 9A, light incident on the APD fromthe bottom surface of the semiconductor substrate 1 a penetrates throughthe semiconductor substrate 1 a and the buffer layer 2 a and is absorbedat the light absorbing layer 3 a, so that a pair of an electron and apositive hole is generated.

[0028] Among the pair of the electron and the positive hole generated inthis way, the electron moves to the n electrode 9 via the semiconductorsubstrate 1 a, and the positive hole is multiplied at the multiplyinglayer 5 a, and moves to the p electrode 8 via the contact layer 6 b.

[0029] In order to make the positive hole be the main carrier among thepair of the electron and the positive hole, a great number of thecarriers of the light absorbing layer 3 a must be electrons.

[0030] Namely, the conductive type of the light absorbing layer 3 a mustbe n type.

[0031] Such a sequential mesa type APD uses a so-called SAM (SeparateAbsorption and Multiplication) structure, in which the multiplying layer5 a and the light absorbing layer 3 a are separated by the electricfield relaxation layer 4 a such that a low electric field intensity isapplied to the light absorbing layer 3 a while a high electric fieldintensity is applied to the multiplying layer 5 a.

[0032] In this case, because the electric field intensity of the n-typelight absorbing layer 3 a is suppressed by the electric field relaxationlayer 4 a, the conductive type of the electric field relaxation layer 4a is the same n type as that of the light absorbing layer 3 a.

[0033] Because such a sequential mesa type APD has a functionavalanche-multiplying the light exciting carrier, the crystallinity ofthe above-described layers is considered to be extremely important.

[0034] Note that, in such a sequential mesa type APD, the epitaxialgrowth itself of each layer can be carried out, in theory, on asemiconductor substrate which is any of an n-type semiconductorsubstrate, a p-type semiconductor substrate, or a semi-isolatedsemiconductor substrate.

[0035] As described above, in the sequential mesa type APD, whenconsidering the fact that light-receiving current flows via thesemiconductor substrate, the semiconductor substrate which is used mustbe an n-type or a p-type semiconductor substrate.

[0036] However, as shown in FIGS. 9A and 9B, because a dopant such asSn, S or the like included in the semiconductor substrate 1 a does notdiffuse during the epitaxial growth, the n-type semiconductor substrate1 a is suitable as a substrate for the epitaxial growth of eachsemiconductor layer.

[0037] On the other hand, in the p-type semiconductor substrate, thereare problems such as the Zn included in the semiconductor substratediffuses during the epitaxial growth, there is the need to form athicker buffer layer by epitaxial growth in order to prevent the Zn fromdiffusing, and because the n-type semiconductor substrate layer isformed by epitaxial growth after the p-type semiconductor substrate isformed by epitaxial growth, the time after the epitaxial growth of thep-type semiconductor layer becomes longer. Thus, diffusion of the Znwhich is the dopant in the p-type semiconductor layer formed by theepitaxial growth easily arises.

[0038] Namely, the p-type semiconductor substrate having such problemsis not generally suitable for a sequential mesa type APD in whichcrystallinity is considered to be extremely important.

[0039] Accordingly, it is preferable that the n-type semiconductorsubstrate 1 a is used as the sequential mesa type APD in order toepitaxially grow a semiconductor layer having good qualitycrystallinity.

[0040] In this way, in order to obtain a good light-receivingcharacteristic in a sequential mesa type APD in which the positive holesare the main carrier and the pn junction is formed by epitaxial growth,the n-type light absorbing layer 3 a and the n type field relaxationlayer 4 a are necessary, and the semiconductor substrate which is usedis preferably the n-type semiconductor substrate 1 a.

[0041] Further, as described above, in a sequential mesa type APD inwhich the positive holes are the main carrier and the pn junction isformed by the epitaxial growth, as shown in FIGS. 9A and 9B, the p-typecontact layer 6 b is used in order to ensure an ohmic electrode in the pelectrode 8.

[0042] At the time of epitaxial growth of the contact layer 6 b, thecontact layer 6 b is doped to p type by using a p-type dopant such as Znor the like.

[0043] Note that, in order to obtain the ohmic electrode, the p-typecarrier density of the contact layer 6 b is preferably set to be as highas possible, for example, about 5×10¹⁸ (cm⁻³) or more.

[0044] Note that the above-described MOVPE method or the like is mainlyused as a growth method (manufacturing method) of the contact layer 6 b.

[0045] Further, due to the Zn which is the dopant of the contact layer 6b being diffused in the n-type electric field relaxation layer 4 a, theconductive type of the multiplying layer 5 a is made to be n type sothat the appropriate internal electric field intensity distribution inthe direction perpendicular to the n-type semiconductor substrate 1 a isnot destroyed.

[0046] Accordingly, the pn junction in the sequential mesa type APD isformed by the p-type contact layer 6 b and the n-type multiplying layer5 a.

[0047] Note that, in this case, the carrier density of the p-typecontact layer 6 b is particularly high as compared with the carrierdensity of the n-type multiplying layer 5 a.

[0048] Therefore, it is ideal that the sequential mesa type APD, inwhich the positive holes are used as the main carrier and the pnjunction is formed by epitaxial growth, has the structure shown in FIGS.9A and 9B.

[0049] Namely, because the sequential mesa type APD basically does notuse a Zn diffusing process to be described later, there is the advantagethat the manufacturing process (the process steps) can be simplified.

[0050] Further, because the sequential mesa type APD uses an n-typesemiconductor in the electric field relaxation layer 4 a which isdifficult to be manufactured by a p-type semiconductor, there is theadvantage that MOVPE, which can epitaxially grow at the wafer asemiconductor layer having high crystallinity, can be used as the methodof manufacturing the sequential mesa type APD.

[0051] Next, a sequential mesa type APD, which has a sequential mesastructure and in which positive holes are used as the main carrier andthe pn junction is formed by Zn diffusion, will be described.

[0052] The structure itself of such a sequential mesa type APD is thesame as the structure of the sequential mesa type APD shown in FIGS. 9Aand 9B.

[0053] As described above, in order to acquire excellent characteristicsat the sequential mesa type APD in which the positive holes are the maincarrier, the n-type light absorbing layer 3 a and the n-type electricfield relaxation layer 4 a are necessary, and it is preferable to usethe n-type semiconductor substrate 1 a. This is also true in the case ofa sequential mesa type APD in which the pn junction is formed by Zndiffusion, and in the case of the above-described sequential mesa typeAPD, in which the pn junction is formed by epitaxial growth.

[0054] Further, the contact layer 6 b is made to be p type by diffusingZn therein by a Zn diffusion method in order to ensure an ohmicelectrode in the p electrode 8.

[0055] Note that, in order to obtain the ohmic electrode, the p-typecarrier density of the contact layer 6 b is preferably set to be as highas possible, for example, about 5×10¹⁸ (cm⁻³) or more.

[0056] Further, in the Zn diffusing method, by heating the Zn rawmaterial and the wafer contained in a container filled with an inert gasatmosphere, the Zn is diffused from the surface of the wafer to theinterior of the wafer.

[0057] At this time, in order to carry out sufficient Zn diffusion,there is the need to control the gas pressure of the inert gasatmosphere so as to maintain a relatively high value by using anexclusively-used controller, and there is the problem that themanufacturing process (process steps) is complicated.

[0058] The Zn diffused in this way remains in the contact layer 6 b, andthe p-type carrier density is enhanced to a degree at which an ohmicelectrode can be obtained, for example, to 5×10¹⁸ (cm⁻³) or more.

[0059] Note that, at this time, because the Zn is not diffused in themultiplying layer 5 a, the conductive type of the multiplying layer 5 ais n type as is.

[0060] In accordance therewith, the pn junction is formed by the p-typecontact layer 6 b, in which the p-type carrier density is increased byZn diffusion, and the n-type multiplying layer 5 a.

[0061] As a result, also in the case of a sequential mesa type APD inwhich positive holes are used the main carrier and the pn junction isformed by Zn diffusion, the structure shown in FIGS. 9A and 9B is ideal.

[0062] Further, the sequential mesa type APD in which the pn junction isformed by Zn diffusion has the advantage that the desired pn junctioncan be formed by appropriately setting the diffusing conditions of theZn.

[0063] Further, the sequential mesa type APD in which the pn junction isformed by Zn diffusion also has the advantage that, because an n-typesemiconductor is used as the electric field relaxation layer 4 a whichis difficult to fabricate by a p-type semiconductor, the MOVPE method,by which a highly crystalline semiconductor layer can be epitaxiallygrown on the wafer, can be used as the manufacturing method.

[0064] On the other hand, because the sequential mesa type APD uses a Zndiffusing process, the sequential mesa type APD has the drawback thatthe manufacturing process (process steps) is complicated due to theabove-described reasons.

[0065] Next, the sequential mesa type APD, which has a sequential mesastructure and in which electrons are used as the main carrier and the pnjunction is formed by epitaxial growth, will be described.

[0066]FIG. 10 shows a cross-sectional view of the sequential mesa typeAPD which has a sequential mesa structure and in which electrons areused as the main carrier and the pn junction is formed by epitaxialgrowth.

[0067] Note that, in this FIG. 10, portions which are the same as thoseof the sequential mesa type APD shown in FIG. 9A are denoted by the samereference numerals.

[0068] Further, an external perspective view of the sequential mesa typeAPD, which is shown in FIG. 10 and in which electrons are used as themain carrier and the pn junction is formed by epitaxial growth, is thesame as in FIG. 9B, and thus, illustration is omitted.

[0069] Namely, as shown in FIG. 10, in the sequential mesa type APD inwhich electrons are used as the main carrier and the pn junction isformed by epitaxial growth, after the n-type buffer layer 2 a, then-type multiplying layer 5 a, the p-type electric field relaxation layer4 b, the p type light absorbing layer 3 b, a p-type window layer 13 b,and the p-type contact layer 6 b are successively formed by epitaxialgrowth on the n-type semiconductor substrate 1 a by using an epitaxialgrowth method, the conical sequential mesa portion 10 is formed bywet-etching from above.

[0070] Further, after the protective layer 7 is coated on the sequentialmesa portion 10, the p electrode 8 contacting the p-type contact layer 6b is formed.

[0071] Further, on the both sides of the sequential mesa portion 10, then electrodes 9 are attached, via the protective layer 11, to anothermesa portion formed for attaching electrodes.

[0072] In such a sequential mesa type APD in which electrons are themain carrier, as shown by the arrow in FIG. 10, light incident from thebottom surface of the semiconductor substrate 1 a penetrates through thesemiconductor substrate 1 a, the buffer layer 2 a, the multiplying layer5 a, and the electric field relaxation layer 4 b and is absorbed at thelight absorbing layer 3 b, so that a pair of an electron and a positivehole is generated.

[0073] Among the pair of the electron and the positive hole generated inthis way, the electron is multiplied at the multiplying layer 5 a andmoves to the n electrode 9 via the n-type semiconductor substrate 1 a,and the positive hole moves to the p electrode 8 via the contact layer 6b.

[0074] In order to make the positive hole be the main carrier among thepair of the electron and the positive hole, a great number of carriersof the light absorbing layer 3 b must be positive holes.

[0075] Namely, in this case, the conductive type of the light absorbinglayer 3 b must be p type.

[0076] In such a sequential mesa type APD in which electrons are themain carrier, the above-described SAM structure, in which themultiplying layer 5 a and the light absorbing layer 3 b are separated bythe electric field relaxation layer 4 b such that a low electric fieldintensity is applied to the light absorbing layer 3 b while a highelectric field intensity is applied to the multiplying layer 5 a, isused.

[0077] In this case, because the electric field intensity of the p typelight absorbing layer 3 b is suppressed by the electric field relaxationlayer 4 b, the conductive type of the electric field relaxation layer 4b is p type which is the same as that of the light absorbing layer 3 b.

[0078] Further, because such a sequential mesa type APD in whichelectrons are the main carrier has a function avalanche-multiplying thelight exciting carrier, the crystallinity of the above-described layersis considered to be extremely important.

[0079] In order to obtain excellent crystallinity of each semiconductorlayer, for the same reasons as in the case of the sequential mesa typeAPD described in FIGS. 9A and 9B in which positive holes are the maincarrier, the semiconductor substrate which is used is preferably then-type semiconductor substrate 1 a.

[0080] Moreover, in order to improve the accuracy of the electric fieldintensity distribution in the sequential mesa portion 10 in thedirection perpendicular to the semiconductor substrate 1 a, because thepn junction is preferably formed between the p-type electric fieldrelaxation layer 4 b and the multiplying layer 5 a, the multiplyinglayer 5 a is n type.

[0081] Such a formed position of the pn junction is also preferable formaking estimation of the amount of decrease in the electric fieldintensity in the multiplying layer 5 a be unnecessary.

[0082] Accordingly, in the sequential mesa type APD in which electronsare the main carrier, the pn junction is formed by the p-type electricfield relaxation layer 4 b and the multiplying layer 5 a.

[0083] In this way, in order to obtain excellent light-receivingcharacteristics in a sequential mesa type APD in which the electrons arethe main carrier and the pn junction is formed by epitaxial growth, thep type light absorbing layer 3 b, the p-type electric field relaxationlayer 4 b, and the n-type multiplying layer 5 a are necessary, and thesemiconductor substrate which is used is preferably the n-typesemiconductor substrate 1 a.

[0084] In such a sequential mesa-type APD, the window layer 13 b also isnecessary in order to prevent the electrons which are a light excitingcarrier from diffusing/moving to the contact layer 6 b.

[0085] Note that GS-MBE (gas-molecule beam epitaxy), MBE (molecule beamepitaxy), and the like are mainly used as the epitaxial growth method.

[0086] Further, in order to ensure the ohmic electrode of the pelectrode 8, the conductive type of the contact layer 6 b is p type.

[0087] Moreover, at the time of epitaxial growth, the contact layer 6 bis doped to a p type by using a p-type dopant such as Be or the like.

[0088] Note that, in order to obtain the ohmic electrode, the p-typecarrier density of the contact layer 6 b is preferably set to be as highas possible, for example, about 5×10¹⁸ (cm⁻³) or more.

[0089] Accordingly, in a sequential mesa type APD in which electrons arethe main carrier, the structure shown in FIGS. 9A and 9B is ideal, andbecause electrons having a light effective mass are the main carrier,there is the feature that it is advantageous with respect to the pointof high-speed performance.

[0090] However, in the APDs having the sequential mesa structures shownin FIGS. 9A, 9B and FIG. 10, there are still the following problemswhich must be improved.

[0091] Firstly, in the sequential mesa type APD in which positive holesare the main carrier, or also in the sequential mesa type APD in whichelectrons are the main carrier, there is the problem that, in eachsemiconductor layer forming the sequential mesa portion 10, except forthe case of selectively diffusing Zn at a specific portion in thesurface parallel to the semiconductor substrate, it is difficult for thein-surface distribution of electric field intensity in a surfaceparallel to the semiconductor substrate to concentrate at the centralportion of the mesa by only the epitaxial growth process.

[0092]FIG. 3 shows measured results of the light-receiving sensitivitydistribution characteristic of a sequential mesa type APD whoselight-receiving diameter is 40 μm.

[0093] Concretely, FIG. 3 shows measured values of light-receivingcurrent (μA) obtained between the p electrode 6 and the n electrode 9 ateach position (μm) in a case in which the irradiating position of anextremely thin light beam is successively moved within theaforementioned range of 40 μm.

[0094] In FIG. 3, characteristic B shows the light-receiving sensitivitydistribution characteristic of the sequential mesa type APD as shown inFIGS. 9A and 9B.

[0095] As illustrated, characteristic B is a double-peakedcharacteristic in which the light-receiving current at the peripheralportion of the mesa shown by the positions −20 μm, +20 μm from thecentral position (0) is larger than the light-receiving current at thecentral portion of the mesa.

[0096] A sequential mesa type APD whose light-receiving characteristicis a double-peaked characteristic in this way has the problem that it isdifficult to align the optical axes at the time of actual use when madeinto a module, and the yield of the modularization deteriorates. Becausealignment of the optical axes must be carried out at the central portionof the mesa at which the light-receiving current is smaller than that ofthe peripheral portion of the mesa, a sufficient light-receivingcharacteristic cannot be exhibited. In addition, it is difficult torealize high sensitization by keeping to a minimum the effects of thedark current and noise contained in the light-receiving signal relatingto the problem of crystallinity described later, and to decrease thefabricating costs of modularization.

[0097] Hereinafter, reasons why these problems arise will be described.

[0098] Because the APD shown in FIGS. 9A and 9B is a sequential mesatype structure, the more the electric field intensity increases, themore the carrier of the positive holes or the electrons is multiplied.

[0099] Accordingly, the magnitude of the light-receiving current showsthe magnitude of the electric field intensity at the pn junction.

[0100] It can be said that the electric field intensity at the peripheryof the mesa is higher and the electric field intensity at the centralportion of the mesa is low in the sequential mesa type APD shown inFIGS. 9A and 9B.

[0101]FIG. 11 shows the way of broadening (width) of the depletionregion (depletion layer) by built-in potential from the pn junction inthe sequential mesa type APD shown in FIGS. 9A and 9B in which positiveholes are used as the main carrier.

[0102] Note that, as described above, because the carrier density of thep-type contact layer 6 b forming the pn junction is higher than thecarrier density of the multiplying layer 5 a, the majority of thedepletion region (depletion layer) is formed at the semiconductorsubstrate 1 a side of the pn junction.

[0103] As shown in FIG. 11, because this APD has a sequential mesastructure, the ratio of the cross-sectional area showing the depletionregion of the p-type contact layer 6 b structuring the pn junction andthe cross-sectional area showing the depletion region of the n-typemultiplying layer 5 a greatly differs at the central portion of the mesaand at the peripheral portion of the mesa.

[0104] Here, considering from the standpoint of depleting the pnjunction portion, because the APD has a sequential mesa structure, atthe vicinity of the periphery of the mesa, there is a state in which thecarrier density of the multiplying layer 5 a is substantially higherthan at the central portion of the mesa.

[0105] In contrast, at the contact layer 6 b, conversely, there is astate in which the carrier density is weak. However, because the carrierdensity is originally high at the contact layer 6 b, even if it is in astate in which the carrier density is substantially weak, the effect issmall.

[0106] As a result, in the sequential mesa type APD, the way ofbroadening (width) of the depletion region is shorter (narrower) thanthe way of broadening (width) of the central portion.

[0107] Namely, it can be understood that the electric field intensity atthe peripheral portion of the mesa is higher than that at the centralportion of the mesa in the sequential mesa type APD.

[0108]FIG. 12 shows the way of broadening (width) of the depletionregion (depletion layer) by built-in potential from the pn junctionportion in the sequential mesa type APD as shown in FIG. 10 in whichelectrons are used as the main carrier.

[0109] In this sequential mesa type APD, the carrier density of thep-type electric field relaxation layer 4 b forming the pn junction ishigher than the carrier density of the multiplying layer 5 a. Thus, asshown in FIG. 12, in accordance with the principles of chargeneutrality, the way of broadening (width) of the depletion region at thevicinity of the periphery of the mesa is shorter (narrower) than the wayof broadening (width) of the central portion.

[0110] Namely, in the sequential mesa type APD, the electric fieldintensity at the peripheral portion of the mesa is higher than that atthe central portion of the mesa.

[0111] The reason for this is that, in the sequential mesa type APD, itis difficult for the in-surface distribution of field intensity in asurface parallel to the semiconductor substrate to concentrate at thecentral portion of the mesa by only the epitaxial growth process, sothat there is a double-peaked characteristic in which thelight-receiving current at the peripheral portion of the mesa is greaterthan the light-receiving current at the central portion of the mesa.

[0112] In this way, in the sequential mesa type APD in which positiveholes or electrons are used as the main carrier and the pn junction isformed by epitaxial growth, the way of broadening (width) of thedepletion region at the vicinity of the periphery of the mesa is shorter(narrower) that at the central portion, and the electric field intensityat the peripheral portion of the mesa is higher than at the centralportion of the mesa.

[0113] Here, the relationship between the crystallinity and thelight-receiving characteristic of the sequential mesa type APD will bedescribed.

[0114] As described above, a sequential mesa type APD of this type, thelight-receiving current is multiplied by an avalanche multiplyingfunction.

[0115] Further, the noise at the time of the avalanche multiplyingfunction greatly depends on the crystallinity of the sequential mesatype APD.

[0116] Accordingly, even among light-receiving elements in whichcrystallinity is considered to be important, in particular, thecrystallinity of a sequential mesa type APD is important.

[0117] In a sequential mesa type APD, a mesa side surface 10 a formed bymesa-etching is provided at the peripheral portion of the mesa of asequential mesa portion 10.

[0118] Generally, the mesa side surface 10 a has a great number ofcrystal defects as compared with the interior portion of the mesa.

[0119] Further, the crystal defects adversely affect the considerationof solutions for decreasing dark current in the sequential mesa typeAPD, decreasing noise, high sensitization, and modularization.

[0120] Namely, in a sequential mesa type APD in which positive holes orelectrons are used as the main carrier and the pn junction is formed byepitaxial growth, as shown by characteristic B of FIG. 3, when thelight-receiving characteristic of the sequential mesa type APD isdominant at the peripheral portion of the mesa, the good crystallinitywhich the central portion of the mesa has is not reflected in thelight-receiving characteristic of the entire sequential mesa type APD.As a result, it is a cause for the light-receiving characteristic of theentire sequential mesa type APD to deteriorate, and for it to bedifficult to align optical axes at the time of making the APD a module,and for the yield of modularization to be poor, and for the fabricatingcosts of modularization to increase.

BRIEF SUMMARY OF THE INVENTION

[0121] An object of the present invention is to provide a sequentialmesa type avalanche photodiode which is achieved on the basis of theabove-described circumstances, and in which, in a sequential mesa typeAPD in which positive holes or electrons are used as the main carrierand a pn junction is formed by epitaxial growth, by making thedistribution of the electric field concentrate at the central portion ofthe mesa, the effects of dark current and noise contained in alight-receiving signal can be kept to a minimum, and high sensitizationcan be realized, and the fabricating costs at the time of modularizationof the APD can be greatly decreased.

[0122] Another object of the present invention is to provide a method ofmanufacturing a sequential mesa type avalanche photodiode which isachieved on the basis of the above-described circumstances, and inwhich, in a sequential mesa type APD in which positive holes orelectrons are used as the main carrier and a pn junction is formed byepitaxial growth, by making the distribution of the electric fieldconcentrate at the central portion of the mesa, the effects of the darkcurrent and noise contained in a light-receiving signal can be kept to aminimum, and high sensitization can be realized, and the fabricatingcosts at the time of modularization of the APD can be greatly decreased.

[0123] First, the point of interest of the present invention will bedescribed.

[0124] As described above, in a sequential mesa type APD in which the pnjunction is formed by only an epitaxial growth process, it is difficultto concentrate, at the central portion of the mesa, the in-surfacedistribution of the electric field intensity in a surface parallel tothe semiconductor substrate.

[0125] Therefore, conventionally, regardless of the fact that there isthe difficulty that the manufacturing process (process steps) iscomplicated, the sequential mesa type APD, in which the pn junction isformed by using the Zn diffusion process which can make the distributionof the electric field concentrate at the central portion of the mesa byselectively diffusing the Zn at a specific portion in a surface parallelto the semiconductor substrate, is exclusively used.

[0126] The present inventor has used in combination contrivances forconcentrating the distribution of the electric field at the centralportion of the mesa which has not been carried out in the prior art, ina sequential mesa type APD in which positive holes or electrons are usedas the main carrier and the pn junction is formed by only an epitaxialgrowth process.

[0127] From the concept opposite that of a sequential mesa type APD inwhich the pn junction is formed by using the conventional Zn diffusionprocess, the present inventor, as a result of diligently searching forsuch contrivances, has found that it suffices that the carrier densityof a semiconductor layer which is near to the semiconductor substrateamong a pair of semiconductor layers forming the pn junction is largerthan the carrier density of a semiconductor layer which is far from thesemiconductor substrate among the pair of semiconductor layers.

[0128] In a sequential mesa type APD structured by satisfying such arelationship, as described above, the ratio of the cross-sectional areasof the pair of semiconductor layers structuring the pn junction formedwithin the mesa portion is constant at the central portion of the mesa,and is different at the central portion of the mesa and at theperipheral portion of the mesa.

[0129] Here, considering from the standpoint of depleting the pnjunction portion, because the APD has a sequential mesa structure, amongthe pair of semiconductor layers structuring the pn junction, at thevicinity of the periphery of the mesa, there is a state in which thecarrier density of the semiconductor layer which is far from thesemiconductor substrate is substantially weaker than at the centralportion of the mesa.

[0130] In contrast, at the semiconductor layer which is near to thesemiconductor substrate, conversely, there is a state in which thecarrier density is high. However, because the carrier density isoriginally high at the semiconductor layer which is near to thesemiconductor substrate, even if it is in a state in which the carrierdensity is substantially high, the effect is small.

[0131] Namely, the way of broadening (width) of the depletion region atthe peripheral portion of the mesa is greater than the way of broadening(width) of the depletion region at the central portion of the mesa, andthe electric field intensity at the central portion of the mesa isgreater than the electric field intensity at the peripheral portion ofthe mesa.

[0132] Thus, in such a sequential mesa type APD according to the presentinvention, the component at the central portion of the mesa contained inthe overall light-receiving characteristic of the APD can be increased,and the component at the peripheral portion of the mesa can bedecreased.

[0133] Accordingly, in such a sequential mesa type APD according to thepresent invention, the effects of the dark current and noise caused dueto crystal defects which are many at the peripheral portion of the mesacan be kept to a minimum, and decreasing of dark current, decreasing ofnoise, and high sensitization in the overall light-receivingcharacteristic of the APD can be attempted. Further, since the yield ofmodularizing is improved by making the alignment of the optical axes atthe time of modularizing be easy and exact, the fabricating costs ofmodularizing can be greatly decreased.

[0134] In such a sequential mesa type APD, the relationship of themagnitude of the carrier densities of the pair of semiconductor layersforming the pn junction is a relationship opposite to the relationshipof the magnitude of the carrier densities of the pair of semiconductorlayers forming the pn junction in a conventional sequential mesa typeAPD in which the pn junction is formed by a Zn diffusion process.

[0135] Next, the background of the difficulty of the idea of therelationship of the magnitude of the carrier densities of the pair ofsemiconductor layers forming the pn junction will be described.

[0136] Namely, in a conventional sequential mesa type APD in which thepn junction is formed by using a Zn diffusion process, as describedabove, the pn junction is formed by the p-type contact layer 6 b, inwhich the p-type carrier density is made high by Zn diffusion, and then-type multiplying layer 5 a.

[0137] At this time, it is preferable, for the contact layer 6 b aswell, that the p-type carrier density of the contact layer 6 b is set tobe as high as possible, for example, about 5×10¹⁸ (cm⁻³) or more. Thegas pressure of the inert gas atmosphere used for carrying outsufficient Zn diffusion is controlled so as to maintain a relativelyhigh value by using an exclusively-used controller.

[0138] Namely, in the conventional sequential mesa type APD in which thepn junction is formed by using a Zn diffusion process, there is arelationship in which the carrier density of the p-type contact layer 6b (the semiconductor layer which is far from the semiconductorsubstrate) forming the pn junction is larger than the carrier density ofthe n-type multiplying layer 5 a (the semiconductor layer which is nearto the semiconductor substrate).

[0139] Accordingly, in the sequential mesa type APD of the presentinvention, whose relationship is opposite to this relationship and inwhich the pn junction is formed by epitaxial growth, it would not begenerally thought to make the carrier density of the semiconductor layerwhich is near to the semiconductor substrate among the pair ofsemiconductor layers forming the pn junction, larger than the carrierdensity of the semiconductor layer which is far from the semiconductorsubstrate among the aforementioned pair of semiconductor layers.

[0140] Further, in the sequential mesa type APD of the present inventionin which the pn junction is formed by epitaxial growth, thesemiconductor layer which is far from the semiconductor substrate amongthe pair of semiconductor layers forming the pn junction, is formedseparate from and at the lower layer of the p-type contact layer, andthe gas pressure of the inert gas atmosphere used for Zn diffusion forobtaining an ohmic electrode of the contact layer 6 b may be maintainedat a relatively low value.

[0141] Accordingly, in accordance therewith, in the sequential mesa typeAPD of the present invention in which the pn junction is formed byepitaxial growth, it would not be generally thought to make the carrierdensity of the semiconductor layer which is near to the semiconductorsubstrate among the pair of semiconductor layers forming the pnjunction, larger than the carrier density of the semiconductor layerwhich is far from the semiconductor substrate among the aforementionedpair of semiconductor layers.

[0142] In order to achieve the above object, there is provided asequential mesa type avalanche photodiode comprising:

[0143] a semiconductor substrate; and

[0144] a sequential mesa portion formed on the semiconductor substrate,a plurality of semiconductor layers which include a light absorbinglayer and a multiplying layer being laminated by epitaxial growth, inthe sequential mesa portion, and a pair of semiconductor layers whichform a pn junction being included in the plurality of semiconductorlayers, wherein

[0145] the carrier density of a semiconductor layer which is near to thesemiconductor substrate among the pair of semiconductor layers is largerthan the carrier density of a semiconductor layer which is far from thesemiconductor substrate among the pair of semiconductor layers, and

[0146] in accordance therewith, in the sequential mesa type avalanchephotodiode, light-receiving current based on movement of electrons andpositive holes generated in the sequential mesa portion when light isincident from the semiconductor substrate toward the light absorbinglayer is larger at a central portion than at a peripheral portion of thesequential mesa portion.

[0147] According to a second aspect of the present invention, there isprovided a sequential mesa type avalanche photodiode according to thefirst aspect, wherein the semiconductor substrate is structured from ann-type semiconductor substrate, and any of the electrons or the positiveholes are the main carrier.

[0148] According to a third aspect of the present invention, there isprovided a sequential mesa type avalanche photodiode according to thesecond aspect, wherein the n-type semiconductor substrate is asemiconductor substrate formed from n⁺-type InP.

[0149] According to a fourth aspect of the present invention, there isprovided a sequential mesa type avalanche photodiode according to thesecond aspect, wherein the semiconductor layer which is near to then-type semiconductor substrate among the pair of semiconductor layers isan n-type semiconductor layer, and the semiconductor layer which is farfrom the n-type semiconductor substrate among the pair of semiconductorlayers is a p-type semiconductor layer.

[0150] According to a fifth aspect of the present invention, there isprovided a sequential mesa type avalanche photodiode according to thefourth aspect, wherein the n-type semiconductor layer (4 a, 14 a, 15 a)is an n-type electric field relaxation layer (4 a), and the p-typesemiconductor layer (5 b, 14 b, 12 b) is a p-type multiplying layer (5b), and the positive holes are the main carrier.

[0151] According to a sixth aspect of the present invention, there isprovided a sequential mesa type avalanche photodiode according to thefifth aspect, wherein the n-type electric field relaxation layer is anelectric field relaxation layer formed from n⁺-type InP, and the p-typemultiplying layer is a multiplying layer formed from p⁻-type InP.

[0152] According to a seventh aspect of the present invention, there isprovided a sequential mesa type avalanche photodiode according to thefourth aspect, wherein the n-type semiconductor layer is an n-typeelectric field relaxation layer, and the p-type semiconductor layer is ap-type electric field concentration layer, and the positive holes arethe main carrier.

[0153] According to an eighth aspect of the present invention, there isprovided a sequential mesa type avalanche photodiode according to theseventh aspect, wherein the n-type electric field relaxation layer is anelectric field relaxation layer formed from n⁺-type InP, and the p-typeelectric field concentration layer is a electric field concentrationlayer formed from p⁻-type InP.

[0154] According to a ninth aspect of the present invention, there isprovided a sequential mesa type avalanche photodiode according to thefourth aspect, wherein the n-type semiconductor layer is an n-typemultiplying layer, and the p-type semiconductor layer is a p⁻-typeelectric field concentration layer, and the electrons are the maincarrier.

[0155] According to a tenth aspect of the present invention, there isprovided a sequential mesa type avalanche photodiode according to theninth aspect, wherein the n-type multiplying layer is a multiplyinglayer formed from n type InP, and the p-type electric fieldconcentration layer is an electric field concentration layer formed fromp⁻-type InP.

[0156] According to an eleventh aspect of the present invention, thereis provided a sequential mesa type avalanche photodiode according to thefourth aspect, wherein the n-type semiconductor layer is an n-typeelectric field concentration layer, and the p-type semiconductor layeris a p-type electric field concentration layer, and the electrons arethe main carrier.

[0157] According to a twelfth aspect of the present invention, there isprovided a sequential mesa type avalanche photodiode according to theeleventh aspect, wherein the n-type electric field concentration layeris a first electric field concentration layer formed from n⁺-type InP,and the p-type electric field concentration layer is a second electricfield concentration layer formed from p⁻-type InP.

[0158] According to a thirteenth aspect of the present invention, thereis provided a sequential mesa type avalanche photodiode according to thefirst aspect, wherein the semiconductor substrate is formed from ap-type semiconductor substrate, and any of the electrons and thepositive holes are the main carrier.

[0159] According to a fourteenth aspect of the present invention, thereis provided a sequential mesa type avalanche photodiode according to thethirteenth aspect, wherein the p-type semiconductor substrate is asemiconductor substrate formed from p⁺-type InP.

[0160] According to a fifteenth aspect of the present invention, thereis provided a sequential mesa type avalanche photodiode according to thethirteenth aspect, wherein a semiconductor layer which is near to thep-type semiconductor substrate among the pair of semiconductor layers isa p-type semiconductor layer, and a semiconductor layer which is farfrom the p-type semiconductor substrate among the pair of semiconductorlayers is an n-type semiconductor layer.

[0161] According to a sixteenth aspect of the present invention, thereis provided a sequential mesa type avalanche photodiode according to thefifteenth aspect, wherein the p-type semiconductor layer is a p-typecontact layer, and the n-type semiconductor layer is an n-typemultiplying layer, and the positive holes are the main carrier.

[0162] According to a seventeenth aspect of the present invention, thereis provided a sequential mesa type avalanche photodiode according to thesixteenth aspect, wherein the p-type contact layer is a contact layerformed from p⁺-type InGaAs, and the n-type multiplying layer is amultiplying layer formed from n⁻-type InP.

[0163] According to an eighteenth aspect of the present invention, thereis provided a sequential mesa type avalanche photodiode according to thefifteenth aspect, wherein the p-type semiconductor layer is a p-typeelectric field relaxation layer, and the n-type semiconductor layer isan n-type multiplying layer, and the electrons are the main carrier.

[0164] According to a nineteenth aspect of the present invention, thereis provided a sequential mesa type avalanche photodiode according to theeighteenth aspect, wherein the p-type electric field relaxation layer isan electric field relaxation layer formed from p⁺-type InP, and then-type multiplying layer is a multiplying layer formed from n⁻-type InP.

[0165] In order achieve the above object, according to a twentiethaspect of the present invention, there is provided a method ofmanufacturing a sequential mesa type avalanche photodiode, comprisingthe steps of:

[0166] preparing a semiconductor substrate;

[0167] laminating a plurality of semiconductor layers, including a lightabsorbing layer and a multiplying layer, on the semiconductor substrateby epitaxial growth, a pair of semiconductor layers which form a pnjunction being included in the plurality of semiconductor layers; and

[0168] forming a sequential mesa portion having a sequential mesaportion structure including therein the plurality of semiconductorlayers,

[0169] wherein

[0170] the carrier density of a semiconductor layer which is near to thesemiconductor substrate among the pair of semiconductor layers is largerthan the carrier density of a semiconductor layer which is far from thesemiconductor substrate among the pair of semiconductor layers, and

[0171] in accordance therewith, in the sequential mesa type avalanchephotodiode, light-receiving current based on movement of electrons andpositive holes generated in the sequential mesa portion when light isincident from the semiconductor substrate toward the light absorbinglayer is larger at a central portion than at a peripheral portion of thesequential mesa portion.

[0172] According to a twenty-first aspect of the present invention,there is provided a method of manufacturing a sequential mesa typeavalanche photodiode according to the twentieth aspect, wherein thesemiconductor substrate is formed from an n-type semiconductorsubstrate, and any of the electrons or the positive holes are the maincarrier.

[0173] According to a twenty-second aspect of the present invention,there is provided a method of manufacturing a sequential mesa typeavalanche photodiode according to the twenty-first aspect, wherein then-type semiconductor substrate is a semiconductor substrate formed fromn⁺-type InP.

[0174] According to a twenty-third aspect of the present invention,there is provided a method of manufacturing a sequential mesa typeavalanche photodiode according to the twenty-first aspect, wherein thesemiconductor layer which is near to the n-type semiconductor substrateamong the pair of semiconductor layers is formed from an n-typesemiconductor layer, and the semiconductor layer which is far from then-type semiconductor substrate among the pair of semiconductor layers isformed from a p-type semiconductor layer.

[0175] According to a twenty-fourth aspect of the present invention,there is provided a method of manufacturing a sequential mesa typeavalanche photodiode according to the twenty-third aspect, wherein then-type semiconductor layer is formed from an n-type electric fieldrelaxation layer, and the p-type semiconductor layer is formed from ap-type multiplying layer, and the positive holes are the main carrier.

[0176] According to a twenty-fifth aspect of the present invention,there is provided a method of manufacturing a sequential mesa typeavalanche photodiode according to the twenty-fourth aspect, wherein then-type electric field relaxation layer is an electric field relaxationlayer formed from n⁺-type InP, and the p-type multiplying layer is amultiplying layer formed from p⁻-type InP.

[0177] According to a twenty-sixth aspect of the present invention,there is provided a method of manufacturing a sequential mesa typeavalanche photodiode according to the twenty-third aspect, wherein then-type semiconductor layer is formed from an n-type electric fieldrelaxation layer, and the p-type semiconductor layer is formed from ap-type electric field concentration layer, and the positive holes arethe main carrier.

[0178] According to a twenty-seventh aspect of the present invention,there is provided a method of manufacturing a sequential mesa typeavalanche photodiode according to the twenty-sixth aspect, wherein then-type electric field relaxation layer is an electric field relaxationlayer formed from n⁺-type InP, and the p-type electric fieldconcentration layer is an electric field concentration layer formed fromp⁻-type InP.

[0179] According to a twenty-eighth aspect of the present invention,there is provided a method of manufacturing a sequential mesa typeavalanche photodiode according to the twenty-third aspect, wherein then-type semiconductor layer is formed from an n-type multiplying layer,and the p-type semiconductor layer is formed from a p-type electricfield concentration layer, and the electrons are the main carrier.

[0180] According to a twenty-ninth aspect of the present invention,there is provided a method of manufacturing a sequential mesa typeavalanche photodiode according to the twenty-eighth aspect, wherein then-type multiplying layer is a multiplying layer formed from n type InP,and the p-type electric field concentration layer is an electric fieldconcentration layer formed from p⁻-type InP.

[0181] According to a thirtieth aspect of the present invention, thereis provided a method of manufacturing a sequential mesa type avalanchephotodiode according to the twenty-third aspect, wherein the n-typesemiconductor layer is formed from an n-type electric fieldconcentration layer, and the p-type semiconductor layer is formed from ap-type electric field concentration layer, and the electrons are themain carrier.

[0182] According to a thirty-first aspect of the present invention,there is provided a method of manufacturing a sequential mesa typeavalanche photodiode according to the thirtieth aspect, wherein then-type electric field concentration layer is a first electric fieldconcentration layer formed from n⁺-type InP, and the p-type electricfield concentration layer is a second electric field concentration layerformed from p⁻-type InP.

[0183] According to a thirty-second aspect of the present invention,there is provided a method of manufacturing a sequential mesa typeavalanche photodiode according to the twentieth aspect, wherein thesemiconductor substrate is formed from a p-type semiconductor substrate,and any of the electrons and the positive holes are the main carrier.

[0184] According to a thirty-third aspect of the present invention,there is provided a method of manufacturing a sequential mesa typeavalanche photodiode according to the thirty-second aspect, wherein thep-type semiconductor substrate (1 b) is a semiconductor substrate formedfrom p⁺-type InP.

[0185] According to a thirty-fourth aspect of the present invention,there is provided a method of manufacturing a sequential mesa typeavalanche photodiode according to the thirty-second aspect, wherein asemiconductor layer which is near to the p-type semiconductor substrateamong the pair of semiconductor layers is formed from a p-typesemiconductor layer, and a semiconductor layer which is far from thep-type semiconductor substrate among the pair of semiconductor layers isformed from an n-type semiconductor layer.

[0186] According to a thirty-fifth aspect of the present invention,there is provided a method of manufacturing a sequential mesa typeavalanche photodiode according to the thirty-fourth aspect, wherein thep-type semiconductor layer is formed from a p-type contact layer, andthe n-type semiconductor layer is formed from an n-type multiplyinglayer, and the positive holes are the main carrier.

[0187] According to a thirty-sixth aspect of the present invention,there is provided a method of manufacturing a sequential mesa typeavalanche photodiode according to the thirty-fifth aspect, wherein thep-type contact layer is a contact layer formed from p⁺-type InGaAs, andthe n-type multiplying layer is a multiplying layer formed from n⁻-typeInP.

[0188] According to a thirty-seventh aspect of the present invention,there is provided a method of manufacturing a sequential mesa typeavalanche photodiode according to the thirty-fourth aspect, wherein thep-type semiconductor layer is formed from a p-type electric fieldrelaxation layer, and the n-type semiconductor layer is formed from ann-type multiplying layer, and the electrons are the main carrier.

[0189] According to a thirty-eighth aspect of the present invention,there is provided a method of manufacturing a sequential mesa typeavalanche photodiode according to the thirty-seventh aspect, wherein thep-type electric field relaxation layer is an electric field relaxationlayer formed from p⁺-type InP, and the n-type multiplying layer is amultiplying layer formed from n⁻-type InP.

[0190] Additional objects and advantages of the invention will be setforth in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention may be realized and obtained bymeans of the instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0191] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate presently preferredembodiment of the invention, and together with the general descriptiongiven above and the detailed description of the preferred embodimentgiven below, serve to explain the principles of the invention.

[0192]FIGS. 1A and 1B are a cross-sectional view and an externalperspective view showing a schematic structure of a sequential mesa typeavalanche photodiode according to a first embodiment of the presentinvention;

[0193]FIG. 2 is a view showing the way of broadening (width) of adepletion region by a built-in potential of the sequential mesa typeavalanche photodiode according to the first embodiment;

[0194]FIG. 3 is a graph showing light-receiving distributioncharacteristics of the sequential mesa type avalanche photodiodeaccording to the first embodiment and a conventional avalanchephotodiode;

[0195]FIG. 4 is a cross-sectional view showing a schematic structure ofa sequential mesa type avalanche photodiode according to a secondembodiment of the present invention;

[0196]FIG. 5 is a cross-sectional view showing a schematic structure ofa sequential mesa type avalanche photodiode according to a thirdembodiment of the present invention;

[0197]FIG. 6 is a cross-sectional view showing a schematic structure ofa sequential mesa type avalanche photodiode according to a fourthembodiment of the present invention;

[0198]FIG. 7 is a cross-sectional view showing a schematic structure ofa sequential mesa type avalanche photodiode according to a fifthembodiment of the present invention;

[0199]FIG. 8 is a cross-sectional view showing a schematic structure ofa sequential mesa type avalanche photodiode according to a sixthembodiment of the present invention;

[0200]FIGS. 9A and 9B are a cross-sectional view and an externalperspective view showing a schematic structure of the conventionalsequential mesa type avalanche photodiode;

[0201]FIG. 10 is a cross-sectional view showing a schematic structure ofanother conventional sequential mesa type avalanche photodiode;

[0202]FIG. 11 is a view showing the way of broadening (width) of adepletion region by a built-in potential of the conventional sequentialmesa type avalanche photodiode; and

[0203]FIG. 12 is a view showing the way of broadening (width) of adepletion region by a built-in potential of the other sequential mesatype avalanche photodiode.

DETAILED DESCRIPTION OF THE INVENTION

[0204] Reference will now be made in detail to the presently preferredembodiments of the invention as illustrated in the accompanyingdrawings, in which like reference numerals designate like orcorresponding parts.

[0205] Hereinafter, embodiments of the present invention will bedescribed with reference to the figures.

FIRST EMBODIMENT

[0206]FIG. 1A is a cross-sectional view of a sequential mesa typeavalanche photodiode (APD) according to a first embodiment of thepresent invention.

[0207]FIG. 1B is an external perspective view of the sequential mesatype APD according to the first embodiment of the present invention.

[0208] In FIGS. 1A and 1B, portions which are the same as those of theconventional sequential mesa type APD shown in FIGS. 9A and 9B aredenoted by the same reference numerals, and detailed description of therepeated portions is omitted.

[0209] In the sequential mesa type APD of the first embodiment, apositive hole is used as the main carrier, and a pn junction is formedby epitaxial growth.

[0210] Namely, as shown in FIGS. 1A and 1B, in the sequential mesa typeAPD of the first embodiment, after a buffer layer 2 a formed fromn⁺-type InP, a light absorbing layer 3 a formed from n⁻-type InGaAs, anelectric field relaxation layer 4 a formed from n⁺-type InP, amultiplying layer 5 b formed from p⁻-type InP, and a contact layer 6 bformed from p⁺-type InGaAs are successively formed by epitaxial growthon a semiconductor substrate 1 a formed from n⁺-type InP by using theMOVPE (organometallic vapor phase epitaxial growth) method, for example,a conical sequential mesa portion 10 is formed by wet-etching fromabove.

[0211] After a protective layer 7 is coated on the sequential mesaportion 10, a p electrode 8 contacting the p-type contact layer 6 b isformed.

[0212] Further, on the both sides of the sequential mesa portion 10, nelectrodes 9 are attached, via the protective layer 11, to another mesaportion formed for attaching electrodes.

[0213] Accordingly, in the sequential mesa type APD of the firstembodiment, the pn junction is formed by the electric field relaxationlayer 4 a formed from n⁺-type InP and the multiplying layer 5 b formedfrom p⁻-type InP.

[0214] Further, the carrier density of the electric field relaxationlayer 4 a, which is formed from n⁺-type InP and which is near to then-type semiconductor substrate 1 a, is set to, for example, 1×10¹⁸(cm⁻³), which is larger than the carrier density, for example, 5×10¹⁶(cm⁻³), of the p-type multiplying layer 5 b far from the n-typesemiconductor substrate 1 a.

[0215] In the sequential mesa type APD structured in this way, as shownby the arrow in FIG. 1A, light incident from the bottom surface of thesemiconductor substrate 1 a penetrates through the semiconductorsubstrate 1 a and the buffer layer 2 a and is absorbed at the lightabsorbing layer 3 a, thereby a pair of an electron and an positive holeis generated.

[0216] Among the pair of the electron and the positive hole generated inthis way, the electron moves to the n electrode 9 via the n-typesemiconductor substrate 1 a, and the positive hole is multiplied at themultiplying layer 5 b and moves to the p electrode 8 via the contactlayer 6 b.

[0217] Moreover, in the sequential mesa type APD of the first embodimentwhich is structured in this way, in the sequential mesa portion 10, asdescribed above, the carrier density of the n-type electric fieldrelaxation layer 4 a near to the n-type semiconductor substrate 1 a isset to, for example, 1×10¹⁸ (cm⁻³), which is larger than the carrierdensity, for example, 5×10¹⁶ (cm⁻³), of the p-type multiplying layer 5 bfar from the n-type semiconductor substrate 1 a.

[0218] Therefore, the in-surface distribution of field intensity in asurface parallel to the semiconductor substrate 1 a concentrates at thecentral portion of the mesa.

[0219] Next, the reason why the in-plane distribution of the electricfield intensity concentrates at the central portion of the mesa will bedescribed.

[0220]FIG. 2 shows the way of broadening (width) of the depletion region(depletion layer) by built-in potential from the pn junction in thesequential mesa type APD of the first embodiment in which positive holesare used as the main carrier as shown in FIGS. 1A and 1B.

[0221] Note that, as described above, the ratio of the cross-sectionalareas of the n-type electric field relaxation layer 4 a and the p-typemultiplying layer 5 b forming the pn junction is constant at the centralportion of the mesa. However, the ratio at the central portion of themesa is different from that at the vicinity of the periphery of themesa.

[0222] Here, considering from the standpoint of depleting the pnjunction portion, because the APD has a sequential mesa structure, atthe vicinity of the periphery of the mesa, there is a state in which thecarrier density of the multiplying layer 5 b is substantially weakerthan at the central portion of the mesa.

[0223] In contrast, at the n-type electric field relaxation layer 4 a,conversely, there is a state in which the carrier density is high.However, because the carrier density is originally high at the n-typeelectric field relaxation layer 4 a, even if it is in a state in whichthe carrier density is substantially high, the effect is small.

[0224] Namely, as shown in FIG. 2, the way of broadening (width) of thedepletion region at the peripheral portion of the mesa is larger thanthe way of broadening (width) of the depletion region at the centralportion of the mesa. Thus, the electric field intensity at the centralportion of the mesa is higher than the electric field intensity at theperipheral portion of the mesa.

[0225] Characteristic A in FIG. 3 shows an actually-measuredlight-receiving sensitivity distribution characteristic of thesequential mesa type APD according to the first embodiment.

[0226] Note that the light-receiving diameter of the sequential mesatype APD of the first embodiment is 30 μm.

[0227] As can be understood from the actually-measured characteristic A,the sequential mesa type APD of the first embodiment has a single-peakedcharacteristic in which the light-receiving current at the centralportion of the mesa is larger than the light-receiving current at theperipheral portion of the mesa.

[0228] In the sequential mesa type APD whose light-receivingcharacteristic is a single-peaked characteristic, as described later,alignment of the optical axes is easy when the APD is modularized andused in actuality, and alignment of the optical axes may be carried outat the central portion of the mesa at which the light-receiving currentis larger than that of the peripheral portion of the mesa. Therefore,there are the advantages that a sufficient light-receivingcharacteristic can be exhibited, and high sensitization is realized bykeeping to a minimum the effects of the dark current and noise containedin the light-receiving signal relating to the above-described problem ofcrystallinity.

[0229] Therefore, the component of the mesa central portion, whichcomponent is contained in the overall light-receiving characteristic ofthe sequential mesa type APD of the first embodiment, can be increased,and the component of the peripheral portion of the mesa can bedecreased.

[0230] Accordingly, the sequential mesa type APD according to the firstembodiment can keep to a minimum the effects of the dark current andnoise caused due to crystal defects which are many at the peripheralportion of the mesa including a mesa side surface 10 a, and decreasingof dark current, decreasing of noise, and high sensitization in theoverall light-receiving characteristic of the sequential mesa type APDcan be attempted.

SECOND EMBODIMENT

[0231]FIG. 4 is a cross-sectional view of a sequential mesa typeavalanche photodiode (APD) according to a second embodiment of thepresent invention.

[0232] In FIG. 4, portions which are the same as those of the sequentialmesa type APD as shown in FIG. 1A according to the first embodiment aredenoted by the same reference numerals, and detailed description of therepeated portions is omitted.

[0233] In the sequential mesa type APD of the second embodiment, in thesame way as in the above-described sequential mesa type APD of the firstembodiment, positive holes are used as the main carrier, and the pnjunction is formed by epitaxial growth.

[0234] Namely, as shown in FIG. 4, in the sequential mesa type APD ofthe second embodiment, an electric field concentration layer 14 b, whichis formed from p⁻-type InP and which is a layer for concentratingelectric fields, is provided between the electric field relaxation layer4 a formed from n⁺-type InP and the multiplying layer 5 b formed fromp⁻-type InP in the sequential mesa portion 10.

[0235] Further, the pn junction is formed by the electric fieldrelaxation layer 4 a formed from n⁺-type InP and the electric fieldconcentration layer 14 b formed from p⁻-type InP.

[0236] In the sequential mesa type APD of the second embodiment as well,with respect to the relationship of the magnitude of the carrierdensities of the n-type electric field relaxation layer 4 a and thep-type electric field concentration layer 14 b, the carrier density ofthe n-type electric field relaxation layer 4 a which is near to thesemiconductor substrate 1 a is set to, for example, 1×10¹⁸ (cm⁻³), whichis larger than the carrier density, for example, 5×10¹⁶ (cm⁻³), of thep-type electric field concentration layer 14 b which is far from thesemiconductor substrate 1 a.

[0237] Therefore, in the sequential mesa type APD of the secondembodiment, the distribution of the electric field intensity within themesa surface concentrates at the central portion of the mesa.

[0238] Accordingly, in the sequential mesa type APD of the secondembodiment as well, substantially the same effects as those of thesequential mesa type APD of the previously-described first embodimentcan be obtained.

[0239] Note that, in the sequential mesa type APD of the secondembodiment, with respect to the relationship of the magnitude of thecarrier densities of the electric field concentration layer 14 b formedfrom p⁻-type InP and the multiplying layer 5 b formed from p-type InP,the setting of these carrier densities can be arbitrarily carried outregardless of the convergence of electric fields.

[0240] Therefore, in the sequential mesa type APD of the secondembodiment, there is no problem even if the multiplying layer 5 b formedfrom InP is p⁺-type.

THIRD EMBODIMENT

[0241]FIG. 5 is a cross-sectional view of a sequential mesa typeavalanche photodiode (APD) according to a third embodiment of thepresent invention.

[0242] In FIG. 5, portions which are the same as those of theconventional sequential mesa type APD shown in FIG. 10 are denoted bythe same reference numerals, and detailed description of the repeatedportions is omitted.

[0243] In the sequential mesa type APD of the third embodiment, in thesame way as in the conventional sequential mesa type APD shown in FIG.10, electrons are used as the main carrier, and the pn junction isformed by epitaxial growth.

[0244] Namely, as shown in FIG. 5, in the sequential mesa type APD ofthe third embodiment, the buffer layer 2 a l formed from n ⁺-type InP,the multiplying layer 5 a l formed from n ⁻-type InP, a first electricfield concentration layer 15 a formed from n⁺-type InP, a secondelectric field concentration layer 12 b formed from p⁻-type InP, theelectric field relaxation layer 4 b formed from p⁺-type InP, a lightabsorbing layer 3 b l formed from p ⁻-type InGaAs, a window layer 13 bformed from p-type InP, and the contact layer 6 b formed from p⁺-typeInGaAs are successively formed by epitaxial growth on the semiconductorsubstrate 1 a formed from n⁺-type InP by using the above-described MBE(molecular beam epitaxy) method. Therefore, the sequential mesa portion10 is formed by wet-etching.

[0245] After the protective layer 7 is coated on the sequential mesaportion 10, the p electrode 8 contacting the p-type contact layer 6 b isformed.

[0246] Further, on the both sides of the sequential mesa portion 10, then electrodes 9 are attached, via the protective layer 11, to anothermesa portion formed for attaching electrodes.

[0247] Accordingly, in the sequential mesa type APD of the thirdembodiment, the pn junction is formed between the first electric fieldconcentration layer 15 a formed from n⁺-type InP and the second electricfield concentration layer 12 b formed from p⁻-type InP.

[0248] Further, in the sequential mesa type APD of the third embodimentas well, the carrier density of the first electric field concentrationlayer 15 a l which is formed from n ⁺-type InP which is near to then-type semiconductor substrate 1 a is set to, for example, 1×10¹⁸(cm⁻³), which is larger than the carrier density, for example, 5×10¹⁶(cm⁻³), of the second electric field concentration layer 12 b which isformed from p-type InP and which is far from the n-type semiconductorsubstrate 1 a.

[0249] Therefore, in the sequential mesa type APD of the thirdembodiment, the distribution of the electric field intensity in the mesasurface concentrates at the central portion of the mesa.

[0250] Accordingly, in the sequential mesa type APD of the thirdembodiment as well, substantially the same effects as in the respectivesequential mesa type APDs of the first and second embodiments can beobtained.

FOURTH EMBODIMENT

[0251]FIG. 6 is a cross-sectional view of a sequential mesa typeavalanche photodiode (APD) according to a fourth embodiment of thepresent invention.

[0252] In FIG. 6, portions which are the same as those of theconventional sequential mesa type APD shown in FIG. 10 are denoted bythe same reference numerals, and detailed description of the repeatedportions is omitted.

[0253] In the sequential mesa type APD of the fourth embodiment, in thesame way as in the conventional sequential mesa type APD shown in FIG.10, electrons are used as the main carrier, and the pn junction isformed by epitaxial growth.

[0254] Namely, as shown in FIG. 6, in the sequential mesa type APD ofthe fourth embodiment, the buffer layer 2 a l formed from n ⁺-type InP,the multiplying layer 5 a l formed from n ⁺-type InP, the electric fieldconcentration layer 14 b formed from p⁻-type InP, the electric fieldrelaxation layer 4 b formed from p⁺-type InP, the light absorbing layer3 b formed from p⁻-type InGaAs, the window layer 13 b formed from p-typeInP, and the contact layer 6 b formed from p⁺-type InGaAs aresuccessively formed by epitaxial growth on the semiconductor substrate 1a formed from n⁺-type InP by using the above-described MBE (molecularbeam epitaxy) method. Therefore, the sequential mesa portion 10 isformed by wet-etching.

[0255] After the protective layer 7 is coated on the sequential mesaportion 10, the p electrode 8 contacting the p-type contact layer 6 b isformed.

[0256] Further, on the both sides of the sequential mesa portion 10, theelectrodes 9 are attached, via the protective layer 11, to another mesaportion formed for attaching electrodes.

[0257] Accordingly, in the sequential mesa type APD of the fourthembodiment, the pn junction is formed between the multiplying layer 5 aformed from n⁺-type InP and the electric field concentration layer 14 bl formed from p ⁻-type InP.

[0258] Further, in the sequential mesa type APD of the fourth embodimentas well, the carrier density of the multiplying layer 5 a, which isformed from n⁺-type InP and which is near to the n-type semiconductorsubstrate 1 a, is set to, for example, 5×10¹⁷ (cm⁻³), which is largerthan the carrier density, for example, 5×10¹⁶ (cm⁻³), of the electricfield concentration layer 14 b which is formed from p⁻-type InP andwhich is far from the n-type semiconductor substrate 1 a.

[0259] Therefore, in the sequential mesa type APD of the fourthembodiment, the distribution of the electric field intensity within themesa surface concentrates at the central portion of the mesa.

[0260] Accordingly, in the sequential mesa type APD of the fourthembodiment as well, substantially the same effects as in the respectivesequential mesa type APDs of the first, second, and third embodimentscan be obtained.

FIFTH EMBODIMENT

[0261]FIG. 7 is a cross-sectional view of a sequential mesa typeavalanche photodiode (APD) according to a fifth embodiment of thepresent invention.

[0262] In FIG. 7, portions which are the same as those of the sequentialmesa type APD shown in FIG. 10 and relating to the first embodiment aredenoted by the same reference numerals, and detailed description of therepeated portions is omitted.

[0263] In the sequential mesa type APD of the fifth embodiment, thep-type semiconductor substrate 16 is used as the semiconductorsubstrate, positive holes are used as the main carrier, and the pnjunction is formed by epitaxial growth.

[0264] Namely, as shown in FIG. 7, in the sequential mesa type APD ofthe fifth embodiment, the buffer layer 2 a l formed from p ⁺-type InP,the contact layer 6 b formed from p⁺-type InGaAs, the multiplying layer5 a formed from n⁻-type InP, the electric field relaxation layer 4 aformed from n⁺-type InP, the light absorbing layer 3 a formed fromn⁻-type InGaAs, the window layer 13 a formed from n⁺-type InP, and acontact layer 16 a l formed from n ⁺-type InGaAs are successively formedby epitaxial growth on the semiconductor substrate 1 b formed fromp⁺-type InP by using the above-described MBE (molecular beam epitaxy)method. Thereafter, the sequential mesa portion 10 is formed bywet-etching.

[0265] After the protective layer 7 is coated on the sequential mesaportion 10, the n electrode 9 contacting the n type contact layer 16 ais formed.

[0266] Further, on the both sides of the sequential mesa portion 10, thep electrodes 8 are attached, via the protective layer 11, to anothermesa portion formed for attaching electrodes.

[0267] Accordingly, in the sequential mesa type APD of the fifthembodiment, the pn junction is formed by the contact layer 6 b formedfrom p⁺-type InGaAs and the multiplying layer 5 a formed from n⁻-typeInP formed within the sequential mesa portion 10.

[0268] Further, in the sequential mesa type APD of the fifth embodiment,the carrier density of the contact layer 6 b, which is formed fromp⁺-type InGaAs and which is near to the p-type semiconductor substrate 1b, is set to, for example, 5×10¹⁷ (cm⁻³), which is larger than thecarrier density, for example, 5×10¹⁶ (cm⁻³), of the multiplying layer 5a which is formed from n-type InP and which is far from the p-typesemiconductor substrate 1 b.

[0269] Therefore, in the sequential mesa type APD of the fifthembodiment, the distribution of the electric field intensity within themesa surface concentrates at the central portion of the mesa.

[0270] Accordingly, in the sequential mesa type APD of the fifthembodiment as well, substantially the same effects as in the respectivesequential mesa type APDs of the first through fourth embodiments can beobtained.

[0271] Note that, in the sequential mesa type APD of the fifthembodiment, it is possible to eliminate the contact layer 6 b formedfrom p⁺-type InGaAs, and to form the p electrode 8 directly from thesemiconductor substrate 1 b formed from p⁺-type InP.

[0272] Further, in the sequential mesa type APD of the fifth embodiment,it is possible to eliminate the contact layer 16 a formed from n⁺-typeInGaAs, and to form the n electrode 9 directly from the window layer 13a formed from n⁺-type InP.

SIXTH EMBODIMENT

[0273]FIG. 8 is a cross-sectional view of a sequential mesa typeavalanche photodiode (APD) according to a sixth embodiment of thepresent invention.

[0274] In FIG. 8, portions which are the same as those of the sequentialmesa type APD shown in FIG. 7 and relating to the fifth embodiment aredenoted by the same reference numerals, and detailed description of therepeated portions is omitted.

[0275] In the sequential mesa type APD of the sixth embodiment, thep-type semiconductor substrate 1 b is used as a semiconductor substrate,and electrons are used as the main carrier, and the pn junction isformed by epitaxial growth.

[0276] Namely, as shown in FIG. 8, in the sequential mesa type APD ofthe sixth embodiment, the buffer layer 2 b l formed from p ⁺-type InP,the contact layer 6 b formed from p⁺-type InGaAs, the window layer 13 bformed from p⁺-type InP, the light absorbing layer 3 b formed fromp⁻-type InGaAs, the electric field relaxation layer 4 b l formed from p⁺-type InP, the multiplying layer 5 a l formed from n ⁻-type InP, andthe contact layer 16 a l formed from n ⁺-type InGaAs are successivelyformed by epitaxial growth on the semiconductor substrate 1 b formedfrom p⁺-type InP, and thereafter, the sequential mesa portion 10 isformed by wet-etching.

[0277] After the protective layer 7 is coated on the sequential mesaportion 10, the n electrode 9 contacting the n-type contact layer 16 ais formed.

[0278] On the both sides of the sequential mesa portion 10, the pelectrodes 8 are attached, via the protective layer 11, to another mesaportion formed for attaching electrodes.

[0279] Further, in the sequential mesa type APD of the sixth embodiment,the pn junction is formed between the electric field relaxation layer 4b formed from p⁺-type InP and the multiplying layer 5 a formed fromn⁻-type InP, within the sequential mesa portion 10.

[0280] Moreover, the carrier density of the electric field relaxationlayer 4 b, which is formed from p⁺-type InP and which is near to thep-type semiconductor substrate 1 b, is set to, for example, 5×10¹⁷(cm⁻³), which is larger than the carrier density, for example, 5×10¹⁶(cm⁻³), of the multiplying layer 5 a which is formed from n⁻-type InPand which is far from the p-type semiconductor substrate 1 b.

[0281] Therefore, in the sequential mesa type APD of the sixthembodiment, the distribution of the electric field intensity within themesa surface concentrates at the central portion of the mesa.

[0282] Accordingly, in the sequential mesa type APD of the sixthembodiment as well, substantially the same effects as in the respectivesequential mesa type APDs of the first through fifth embodiments can beobtained.

[0283] Note that, in the sequential mesa type APD of the sixthembodiment, it is possible to eliminate the contact layer 6 b formedfrom p⁺-type InGaAs, and to form the p electrode 8 directly from thesemiconductor substrate 1 b formed from p⁺-type InP.

[0284] Further, it is possible to eliminate the contact layer 16 aformed from n⁺-type InGaAs, and to form the n electrode 9 directly fromthe window layer 13 a formed from n⁺-type InP.

[0285] As described above, in all of the first through sixthembodiments, it is important that, among a pair of semiconductor layersforming the pn junction formed by epitaxial growth in the sequentialmesa portion 10 at the sequential mesa type APD, the carrier density ofthe semiconductor layer which is near to the semiconductor substrates 1a, 1 b is larger than the carrier density of the semiconductor layerwhich is far from the semiconductor substrates 1 a, 1 b, and inaccordance therewith, the distribution of the electric field intensityin a surface of the mesa concentrates at the central portion of themesa.

[0286] Accordingly, in the present invention, except for therelationship of the magnitude of the carrier densities of the pair ofsemiconductor layers forming the pn junction of the sequential mesa typeAPD by epitaxial growth, any semiconductor layer structure can bearbitrarily set.

[0287] As described above, in the sequential mesa type avalanchephotodiode of the present invention, the carrier density of asemiconductor layer which is near to the semiconductor substrate islarger than the carrier density of a semiconductor layer which is farfrom the semiconductor substrate in a pair of semiconductor layersstructuring the pn junction formed by epitaxial growth in the sequentialmesa portion of the avalanche photodiode. Therefore, the light-receivingcurrent based on the movement of the electrons and the positive holesgenerated in the sequential mesa portion when light is incident from theaforementioned semiconductor substrate toward the aforementioned lightabsorbing layer, is larger at the peripheral portion of theaforementioned mesa portion than at the central portion.

[0288] Accordingly, in accordance with the sequential mesa typeavalanche photodiode of the present invention, the distribution of theelectric field intensity in a surface of the mesa concentrates at thecentral portion of the mesa. Therefore, the effects of the dark currentand noise caused due to crystal defects which are many at the peripheralportion of the mesa including a mesa side surface can be kept to aminimum, and decreasing of dark current, decreasing of noise, and highsensitization in the overall light-receiving characteristic of thesequential mesa type avalanche photodiode can be attempted.

[0289] Further, because the electric field concentrates at the centralportion of the mesa, the following great effects can be obtained withrespect to the points of mounting/evaluation of the APD as well.

[0290] First, as shown by characteristic A in FIG. 3, because the APDaccording to the present invention has a single-peaked characteristic inwhich the light-receiving current at the central portion of the mesa islarger than the light-receiving current at the peripheral portion of themesa, there is only one peak of the photoelectric current. Thus, thecenter-adjusting work (the above-described alignment of the opticalaxes), which sets a micromotion platform such that the light from afiber is irradiated onto a light-receiving portion of the APD and thephotoelectric current of the APD is made to be a maximum, can be easilycarried out.

[0291] As a result, the time required for the center-adjusting work canbe greatly shortened as compared with the APD according to the prior artin which a plurality of peaks of photoelectric current existcircumferentially as viewed from above the mesa. Therefore, making thework more efficient can be attempted.

[0292] Further, in the center-adjusting work according to the prior art,there is little photoelectric current at the central portion of the mesaat which the crystallinity is good and there is low noise, and thephotoelectric current is large at the peripheral portion of the mesa atwhich the crystallinity deteriorates and there is much noise. Therefore,it is unclear at which portion of the mesa the sensitivity as a moduleon the communication measured after the APD is modularized, will be amaximum when the light is incident. However, in the APD according to thepresent invention, because the maximum photoelectric current can beobtained at the central portion of the mesa at which the crystallinityis good, at the time of carrying out the center-adjusting work, thealignment position at which the maximum sensitivity as a module oncommunication is obtained can be already known.

[0293] Namely, in the APD according to the prior art, it is easy forerrors in the center-adjusting work, by which it is determined to be adefective good as a result after modularizing, to arise. However, in theAPD according to the present invention, errors in the center-adjustingwork discovered after modularizing do not arise, and yield is improvedover the APD according to the prior art.

[0294] The improvement in the yield can resolve the uncertainty thatmodularizing progresses while it is unclear whether the item is a gooditem or a defective item which is the problem so far, and can greatlydecrease the fabricating costs of modularizing an APD, because the workof modularizing the APD through many processes is made to be reliable.

[0295] As described above in detail, in accordance with the presentinvention, there is provided a sequential mesa type avalanche photodiodein which, in a sequential mesa type APD in which positive holes orelectrons are used as the main carrier and a pn junction is formed byepitaxial growth, by making the distribution of the electric fieldconcentrate at the central portion of the mesa, the effects of the darkcurrent and noise contained in the light-receiving signal can be kept toa minimum, and high sensitization can be realized, and the fabricatingcosts at the time of modularization of the APD can be greatly decreased.

[0296] Further, in accordance with the present invention, there isprovided a method of manufacturing a sequential mesa type avalanchephotodiode in which, in a sequential mesa type APD in which positiveholes or electrons are used as the main carrier and a pn junction isformed by epitaxial growth, by making the distribution of the electricfield concentrate at the central portion of the mesa, the effects of thedark current and noise contained in the light-receiving signal can bekept to a minimum, and high sensitization can be realized, and thefabricating costs at the time of modularization of the APD can begreatly decreased.

[0297] Additional advantages and modifications will readily occur tothose skilled in the art. Therefore, the invention in its broaderaspects is not limited to the specific details and representativeembodiments shown and described herein. Accordingly, variousmodifications may be made without departing from the spirit or scope ofthe general inventive concept as defined by the appended claims andtheir equivalents.

What is claimed is:
 1. A sequential mesa type avalanche photodiodecomprising: a semiconductor substrate; and a sequential mesa portionformed on the semiconductor substrate, a plurality of semiconductorlayers which include a light absorbing layer and a multiplying layerbeing laminated by epitaxial growth, in the sequential mesa portion, anda pair of semiconductor layers which form a pn junction being includedin the plurality of semiconductor layers, wherein the carrier density ofa semiconductor layer which is near to the semiconductor substrate amongthe pair of semiconductor layers is larger than the carrier density of asemiconductor layer which is far from the semiconductor substrate amongthe pair of semiconductor layers, and in accordance therewith, in thesequential mesa type avalanche photodiode, light-receiving current basedon movement of electrons and positive holes generated in the sequentialmesa portion when light is incident from the semiconductor substratetoward the light absorbing layer is larger at a central portion than ata peripheral portion of the sequential mesa portion.
 2. A sequentialmesa type avalanche photodiode according to claim 1, wherein thesemiconductor substrate is structured from an n-type semiconductorsubstrate, and any of the electrons or the positive holes are the maincarrier.
 3. A sequential mesa type avalanche photodiode according toclaim 2, wherein the n-type semiconductor substrate is a semiconductorsubstrate formed from n⁺-type InP.
 4. A sequential mesa type avalanchephotodiode according to claim 2, wherein the semiconductor layer whichis near to the n-type semiconductor substrate among the pair ofsemiconductor layers is an n-type semiconductor layer, and thesemiconductor layer which is far from the n-type semiconductor substrateamong the pair of semiconductor layers is a p-type semiconductor layer.5. A sequential mesa type avalanche photodiode according to claim 4,wherein the n-type semiconductor layer is an n-type electric fieldrelaxation layer, and the p-type semiconductor layer is a p-typemultiplying layer, and the positive holes are the main carrier.
 6. Asequential mesa type avalanche photodiode according to claim 5, whereinthe n-type electric field relaxation layer is an electric fieldrelaxation layer formed from n⁺-type InP, and the p-type multiplyinglayer is a multiplying layer formed from p⁻-type InP.
 7. A sequentialmesa type avalanche photodiode according to claim 4, wherein the n-typesemiconductor layer is an n-type electric field relaxation layer, andthe p-type semiconductor layer is a p-type electric field concentrationlayer, and the positive holes are the main carrier.
 8. A sequential mesatype avalanche photodiode according to claim 7, wherein the n-typeelectric field relaxation layer is an electric field relaxation layerformed from n⁺-type InP, and the p-type electric field concentrationlayer is a electric field concentration layer formed from p⁻-type InP.9. A sequential mesa type avalanche photodiode according to claim 4,wherein the n-type semiconductor layer is an n-type multiplying layer,and the p-type semiconductor layer is a p⁻-type electric fieldconcentration layer, and the electrons are the main carrier.
 10. Asequential mesa type avalanche photodiode according to claim 9, whereinthe n-type multiplying layer is a multiplying layer formed from n typeInP, and the p-type electric field concentration layer is an electricfield concentration layer formed from p⁻-type InP.
 11. A sequential mesatype avalanche photodiode according to claim 4, wherein the n-typesemiconductor layer is an n-type electric field concentration layer, andthe p-type semiconductor layer is a p-type electric field concentrationlayer, and the electrons are the main carrier.
 12. A sequential mesatype avalanche photodiode according to claim 11, wherein the n-typeelectric field concentration layer is a first electric fieldconcentration layer formed from n⁺-type InP, and the p-type electricfield concentration layer is a second electric field concentration layerformed from p⁻-type InP.
 13. A sequential mesa type avalanche photodiodeaccording to claim 1, wherein the semiconductor substrate is formed froma p-type semiconductor substrate, and any of the electrons and thepositive holes are the main carrier.
 14. A sequential mesa typeavalanche photodiode according to claim 13, wherein the p-typesemiconductor substrate is a semiconductor substrate formed from p⁺-typeInP.
 15. A sequential mesa type avalanche photodiode according to claim13, wherein a semiconductor layer which is near to the p-typesemiconductor substrate among the pair of semiconductor layers is ap-type semiconductor layer, and a semiconductor layer which is far fromthe p-type semiconductor substrate among the pair of semiconductorlayers is an n-type semiconductor layer.
 16. A sequential mesa typeavalanche photodiode according to claim 15, wherein the p-typesemiconductor layer is a p-type contact layer, and the n-typesemiconductor layer is an n-type multiplying layer, and the positiveholes are the main carrier.
 17. A sequential mesa type avalanchephotodiode according to claim 16, wherein the p-type contact layer is acontact layer formed from p⁺-type InGaAs, and the n-type multiplyinglayer is a multiplying layer formed from n⁻-type InP.
 18. A sequentialmesa type avalanche photodiode according to claim 15, wherein the p-typesemiconductor layer is a p-type electric field relaxation layer, and then-type semiconductor layer is an n-type multiplying layer, and theelectrons are the main carrier.
 19. A sequential mesa type avalanchephotodiode according to claim 18, wherein the p-type electric fieldrelaxation layer is an electric field relaxation layer formed fromp⁺-type InP, and the n-type multiplying layer is a multiplying layerformed from n⁻-type InP.
 20. A method of manufacturing a sequential mesatype avalanche photodiode, comprising the steps of: preparing asemiconductor substrate; laminating a plurality of semiconductor layers,including a light absorbing layer and a multiplying layer, on thesemiconductor substrate by epitaxial growth, a pair of semiconductorlayers which form a pn junction being included in the plurality ofsemiconductor layers; and forming a sequential mesa portion having asequential mesa portion structure including therein the plurality ofsemiconductor layers, wherein the carrier density of a semiconductorlayer which is near to the semiconductor substrate among the pair ofsemiconductor layers is larger than the carrier density of asemiconductor layer which is far from the semiconductor substrate amongthe pair of semiconductor layers, and in accordance therewith, in thesequential mesa type avalanche photodiode, light-receiving current basedon movement of electrons and positive holes generated in the sequentialmesa portion when light is incident from the semiconductor substratetoward the light absorbing layer is larger at a central portion than ata peripheral portion of the sequential mesa portion.
 21. A method ofmanufacturing a sequential mesa type avalanche photodiode according toclaim 20, wherein the semiconductor substrate is formed from an n-typesemiconductor substrate, and any of the electrons or the positive holesare the main carrier.
 22. A method of manufacturing a sequential mesatype avalanche photodiode according to claim 21, wherein the n-typesemiconductor substrate is a semiconductor substrate formed from n⁺-typeInP.
 23. A method of manufacturing a sequential mesa type avalanchephotodiode according to claim 21, wherein the semiconductor layer whichis near to the n-type semiconductor substrate among the pair ofsemiconductor layers is formed from an n-type semiconductor layer, andthe semiconductor layer which is far from the n-type semiconductorsubstrate among the pair of semiconductor layers is formed from a p-typesemiconductor layer.
 24. A method of manufacturing a sequential mesatype avalanche photodiode according to claim 23, wherein the n-typesemiconductor layer is formed from an n-type electric field relaxationlayer, and the p-type semiconductor layer is formed from a p-typemultiplying layer, and the positive holes are the main carrier.
 25. Amethod of manufacturing a sequential mesa type avalanche photodiodeaccording to claim 24, wherein the n-type electric field relaxationlayer is an electric field relaxation layer formed from n⁺-type InP, andthe p-type multiplying layer is a multiplying layer formed from p⁻-typeInP.
 26. A method of manufacturing a sequential mesa type avalanchephotodiode according to claim 23, wherein the n-type semiconductor layeris formed from an n-type electric field relaxation layer, and the p-typesemiconductor layer is formed from a p-type electric field concentrationlayer, and the positive holes are the main carrier.
 27. A method ofmanufacturing a sequential mesa type avalanche photodiode according toclaim 26, wherein the n-type electric field relaxation layer is anelectric field relaxation layer formed from n⁺-type InP, and the p-typeelectric field concentration layer is an electric field concentrationlayer formed from p⁻-type InP.
 28. A method of manufacturing asequential mesa type avalanche photodiode according to claim 23, whereinthe n-type semiconductor layer is formed from an n-type multiplyinglayer, and the p-type semiconductor layer is formed from a p-typeelectric field concentration layer, and the electrons are the maincarrier.
 29. A method of manufacturing a sequential mesa type avalanchephotodiode according to claim 28, wherein the n-type multiplying layeris a multiplying layer formed from n type InP, and the p-type electricfield concentration layer is an electric field concentration layerformed from p⁻-type InP.
 30. A method of manufacturing a sequential mesatype avalanche photodiode according to claim 23, wherein the n-typesemiconductor layer is formed from an n-type electric fieldconcentration layer, and the p-type semiconductor layer is formed from ap-type electric field concentration layer, and the electrons are themain carrier.
 31. A method of manufacturing a sequential mesa typeavalanche photodiode according to claim 30, wherein the n-type electricfield concentration layer is a first electric field concentration layerformed from n⁺-type InP, and the p-type electric field concentrationlayer is a second electric field concentration layer formed from p⁻-typeInP.
 32. A method of manufacturing a sequential mesa type avalanchephotodiode according to claim 20, wherein the semiconductor substrate isformed from a p-type semiconductor substrate, and any of the electronsand the positive holes are the main carrier.
 33. A method ofmanufacturing a sequential mesa type avalanche photodiode according toclaim 32, wherein the p-type semiconductor substrate (1 b) is asemiconductor substrate formed from p⁺-type InP.
 34. A method ofmanufacturing a sequential mesa type avalanche photodiode according toclaim 32, wherein a semiconductor layer which is near to the p-typesemiconductor substrate among the pair of semiconductor layers is formedfrom a p-type semiconductor layer, and a semiconductor layer which isfar from the p-type semiconductor substrate among the pair ofsemiconductor layers is formed from an n-type semiconductor layer.
 35. Amethod of manufacturing a sequential mesa type avalanche photodiodeaccording to claim 34, wherein the p-type semiconductor layer is formedfrom a p-type contact layer, and the n-type semiconductor layer isformed from an n-type multiplying layer, and the positive holes are themain carrier.
 36. A method of manufacturing a sequential mesa typeavalanche photodiode according to claim 35, wherein the p-type contactlayer is a contact layer formed from p⁺-type InGaAs, and the n-typemultiplying layer is a multiplying layer formed from n⁻-type InP.
 37. Amethod of manufacturing a sequential mesa type avalanche photodiodeaccording to claim 34, wherein the p-type semiconductor layer is formedfrom a p-type electric field relaxation layer, and the n-typesemiconductor layer is formed from an n-type multiplying layer, and theelectrons are the main carrier.
 38. A method of manufacturing asequential mesa type avalanche photodiode according to claim 37, whereinthe p-type electric field relaxation layer is an electric fieldrelaxation layer formed from p⁺-type InP, and the n-type multiplyinglayer is a multiplying layer formed from n⁻-type InP.